1. Field of the Invention
This invention relates to electronic systems that measures the performance of a high-speed data communications channel. More specifically, this invention relates to a method and system that combines the techniques of bit error rate detection with the functions used to create an eye diagram to accurately measure the performance of a high-speed data channel, device or system without the need of additional components.
2. Description of the Related Art
In recent years, the performance of a high-speed communication facility or device has risen to a point that the ability of accurately measure its throughput has become an ever increasing challenge. In the area of data communication devices, for example, efforts to reduce size and power, while increasing the throughput of a device have increased the possibility of error. Network suppliers, integrators and users want assurances that these devices will perform reliably and can accommodate data transmission rates that routinely exceed several gigabits/second. Providing such assurance requires determining the effective error rate of either the data received by or data transmitted through a high speed communications channel or device.
One approach of evaluating the performance of a high speed communications channel device or system is to use a BERT (BERT). Here, a predetermined bit pattern is transmitted through the target channel, device or system and on to BERT. The difference between the transmitted bit pattern and the bit stream generated by the target device or system are considered errors and are accumulated by the BERT to determine the effective bit error rate (i.e., the fraction of the received bits that are in error) of the target device or system. Even though bit error rate testing (BER) is a relatively significant measure of performance, merely measuring the error rate of a high speed communications device or system does not provide enough data to characterize the behavior or determine an acceptable bit error rate of a communications channel, device or system under a number of test conditions. Furthermore, measuring the bit error rate of a high speed communications device or system provides no warning as to when a slight degradation in the performance or throughput of a device or system might occur. More precisely, bit error rate measurements provide only an indication of any particular performance degradation after it has occurred. Of course, at that point the user would have already become aware the performance degradation.
For these reasons, eye diagrams are used in analyzing the behavior and performance characteristics of a high-speed communications channel, channel or system during the various stages of development, manufacture and installation. One approach utilizing an eye diagram includes a sampling oscilloscope to generate a voltage-versus-time plot of a repetitive waveform and displays the results in the form of an eye diagram. Here, the data signal transmitted by the high speed communications channel, device or system and a clocking pulse or trigger synchronized to that of the bit stream of the applied data signal is supplied to the oscilloscope. Samples of the voltage of the applied data signal are then taken at various instances in time with respect to the trigger and plotted as events on the display of the oscilloscope. Voltage samples are continuously taken and added in combination with the older sampled events that have already been displayed. Over a relatively short period of time, a plurality of events is displayed on the oscilloscope showing the possible voltage distribution of the applied data signal. By accumulating the sampled events for all instances in time and for a specified voltage range, an eye diagram is drawn depicting the behavior and performance characteristics of a high-speed communications channel or device.
The problem with using devices such as a sampling oscilloscope to measure the performance characteristics of a high-speed communication device or system is that as the data rate of the transmitted signal increases, the bandwidth of the sampling oscilloscopes required to analyze these signals and generate an eye diagram must increase proportionally which, in turn increases the cost and sophistication of such oscilloscope. In addition, one or more of the methods to sample the bit stream of an incoming data signal only accumulates a relatively small number of samples per second allowing many, many edges of the trigger pulse to go by between taking samples thereby limiting the sampling rate to between one hundred thousand and two hundred thousand samples per second.
To overcome these limitations, it is a common practice to use a BERT in conjunction with a more sophisticated sampling oscilloscope. Using this configuration, an eye diagram can be generated and displayed on the oscilloscope at the same time as the error rate of a target device or system is measured by the BERT. Thus, by incorporating the functionality of generating an eye diagram into the testing process, a quicker and a more accurate measure of the behavior and performance of a communication device or system can be made at any time while a bit error measurement is taking place. In addition, the above combination also provides the ability to easily spot design or manufacturing defects in more complex communication devices or systems that might not have been detected by just using a bit error rate testing process, alone. However, the disadvantage with this approach is the cost of these two highly sophisticated instruments as well as the duplication of components that perform similar functions to analyze the behavior and performance of a high-speed communication device or system.
Therefore, it is desirable to utilize the same components and signal detection path of binary data bit decision mechanism such as a BERT used to obtain the effective error rate of an incoming data signal to generate an eye diagram that accurately depicts the behavior and performance characteristics of a high-speed data channel, device or system.